1. Field of the Invention
The present invention relates to a broadband reflection type brightness enhancement polarizer provided with a cholesteric liquid crystal film laminate in which a plurality of cholesteric liquid crystal films having selectively reflecting wavelength ranges different from each other are laminated, and a liquid crystal display having the polarizer.
2. Description of the Related Art
A display panel is broadly used as a display of electronic calculators, electronic watches or clocks, automobile navigators, office automation equipments, notebook computers, information communication terminals and so on. For example, a liquid crystal device is configured so that liquid crystal is injected between upper and lower panels that have a transparent electrode and an alignment layer therein to face with each other. A currently used liquid crystal device employs a linearly polarized light, so absorptive polarizers are arranged before and after a panel.
The absorptive polarizer is generally prepared by adsorbing iodine or dichroism die to a polyvinyl alcohol film and then elongating it. The polarizer prepared as mentioned above however has a weak mechanical strength in a direction of a transmission axis and is easily contracted due to heat or moisture, which results in serious deterioration of its polarizing function. Thus, this absorptive polarizer is attached between supports such as cellulose acetate films, when being used. The absorptive polarizer employing polyvinyl alcohol as mentioned above gives linearly polarized lights by absorbing light vibrating in one direction but transmitting light vibrating in another direction, so the efficiency of the polarizer cannot exceed 50% theoretically. It will be a main factor of deteriorating efficiency and brightness of LCD.
Meanwhile, if a reflective polarizer prepared using a cholesteric liquid crystal is additionally used between the reflector and the liquid crystal panel provided with the absorptive polarizer, the above drawbacks of the absorptive polarizer can be solved. The cholesteric liquid crystal is configured so that its liquid crystal is twisted in a spiral direction identical to a circular polarization direction, and it has a selective reflecting characteristic, which reflects only a light having circular polarization whose wavelength is identical to the spiral pitch of the liquid crystal. When using this selective reflecting characteristic, it is possible to make a polarizer that can convert a non-polarized light in a certain wavelength range into a circularly polarized light. That is to say, if a non-polarized light in which left and right circularly polarized lights are mixed half and half is put into a cholesteric liquid crystal film having a spiral structure rotating to the left or right, a circularly polarized light rotating in the same direction as the spiral direction is reflected and a circularly polarized light rotating in the opposite direction is transmitted. At this time, the transmitted circularly polarized light is changed into a linearly polarized light with passing through a film having ¼λ phase difference. Meanwhile, the reflected circularly polarized light changes its polarization direction when being reflected again on the reflector, so it passes through the liquid crystal film. Thus, if the polarizer prepared using a cholesteric liquid crystal film is additionally used, there is theoretically no loss of light, so brightness can be remarkably improved.
However, a backlight used in the liquid crystal device generates a light in a visual ray region (400 to 700 nm) that mainly gives colors, so the selective reflecting region of the cholesteric liquid crystal film should cover the entire visual ray region. If not, there exists a light transmitting through the polarizer in a non-polarized state, thereby deteriorating quality of the liquid crystal device. One cholesteric liquid crystal film cannot cover the entire visual ray region, so a plurality of cholesteric liquid crystal films having selective reflecting wavelength ranges different from each other are laminated to make the polarizer as disclosed in Korean Laid-open Patent Publication No. 1999-65280. However, commonly used cholesteric liquid crystal compounds have narrow selective reflecting wavelength ranges, so at least four cholesteric liquid crystal films should be laminated to cover the entire visual ray region. Accordingly, this technique has drawbacks that a polarizer manufacturing process is more complicated, its economic benefit is reduced, and the made polarizer has greater thickness, which deteriorates the brightness characteristic.